The performance of an engine is determined by the quality of its components, in particular that of the spark plugs. Spark plugs represent one of the most heavily used core components in the engine compartment which, due to the electrical and physical forces acting on them when the fuel mixture is ignited, are subject to increased wear compared to other components.
Spark plugs are essentially composed of a ceramic housing and the two electrodes, the ground electrode and the central electrode. The actual layers (referred to as electrode faces hereinafter) are applied to the opposing faces of the particular electrode in the form of pins for producing a spark, which are mostly made of an expensive noble metal alloy. There is a gap, known as spark gap, between the two electrode faces.
At ignition, a high voltage is generated periodically in the spark gap by an ignition system controlled by the engine, which discharges in a sparkover between the two electrode faces of the spark plug. The spark generated then ignites the compressed air-fuel mixture. Since the sensitive electrode faces are exposed to extremely high temperatures every time a spark is generated, the individual components of the spark plug must be optimally adjusted to ensure a long life even in extended use.
Spark plugs having a laterally oriented ground electrode may be used instead of those in which the ground electrode covers the central electrode from above because the air-fuel mixture may better reach the spark gap between the two electrode faces. However, problems arise in manufacturing an electrode system having a laterally oriented ground electrode. To orient the ground electrode laterally with respect to the central electrode, the ground electrode must be mechanically bent into the correct shape. Furthermore, to make it possible to apply the pin correctly to the electrode front side, the pin should have a certain minimum length. Otherwise the two faces to be attached to each other cannot be optimally welded together. The sum of the lengths of the pin and the electrode yields the total length of the ground electrode. However, since the distance to the central electrode is strongly limited and, in addition, it spatially shields the location where the ground electrode should be placed, the ground electrode must be relatively short, so that, after being bent into the correct shape, a sufficient spark gap still remains between the electrode faces of the ground electrode and the central electrode. Controlled, defined bending of a short, solid piece of metal is the more difficult the shorter is the piece of metal to be bent.
European patent document EP 0 765 017 A1 discusses a spark plug electrode which includes a cylindrical central electrode and two ground electrodes, each ground electrode having a curved front side whose curvature approximately corresponds to the reciprocal of the curvature of the cylindrical central electrode. This should ensure that the distance between the two electrode surfaces is the same along the entire electrode surfaces, so that the spark discharge acts on the entire electrode surface, rather than on only some areas. A spark plug of this type is, however, very complex and expensive to manufacture.